1. Field of the Invention
This invention relates to a high-voltage and large-capacity thyristor valve constituted by a plurality of thyristors in series-parallel connection.
2. Description of the Related Art
FIG. 4 is a circuit diagram of a thyristor valve used in a typical rectifier circuit. In FIG. 4, 1-1 to 1-6 denote thyristor valves which are triggered directly by a light signal, for example, and 2-1 to 2-6 denote overvoltage protection arresters, or protection elements such as nonlinear resistors (hereinafter referred to as arresters), which are respectively connected in parallel with the thyristor valves valve arrester. Further, 3 denotes a conversion transformer, and 4 denotes a D.C. reactor.
The thyristor valves with the construction described above will be subjected to an overvoltage in the following cases:
An overvoltage, caused by accident or lightning stroke in the primary side of converter transformer 3 of FIG. 4, is transmitted to the thyristor valve, via converter transformer 3.
An overvoltage is transmitted from D.C. side of the thyristor valve.
In either cases, an overvoltage applied to the thyristor valve is limited by protection level VP of arrester 2, and it will not exceed VP.
Therefore, it is necessary to take the following two conditions into consideration for designing the thyristor valve.
(1) The thyristor valve can withstand the maximum overvoltage or protection level VP of the valve arrester while it is kept in the nonconductive state.
(2) The thyristor valve can withstand the stress caused when it is turned on while the arrester is in operation, that is, when it is turned on while the arrester is operated and the thyristor valve voltage is clipped at VP after an overvoltage is applied to the thyristor valve.
Since, in the case (2), there is no correlation between the overvoltage of the thyristor valve and the gate pulse, the thyristor valve may actually be turned on while the arrester is operated.
FIGS. 7A and 7B show the result of detail analysis made by the inventor of this invention using an equivalent circuit of FIG. 6 with respect to the stress caused in the case (2). (It should be noted that the following analysis is not publicly known.) FIG. 7A shows the relation between voltage (Vs), current (i), and time, and FIG. 7B shows the relation between time and junction temperature rises Tj1 and Tj6 in the thyristor.
For reference, FIGS. 8A and 8B show the result of analysis of the stress caused at the time of turn-on of the thyristor valve when the arrester operation is not effected (c or the arrester is not provided).
In FIG. 6, Lo denotes a commutation reactance, E an overvoltage source, iA an arrester current, i1 to i6 thyristor currents, and Vs1 to Vs6 thyristor voltages. In the example of calculation, six series-connected thyristors are provided, five thyristors (10-2 to 10-6) out of them are turned on at the same time, and the remaining one is turned on with a delay time of 0.85 .mu.s.
The following fact will be found from the results shown in FIGS. 7 and 8.
In the case shown in FIG. 7 (in the case of turn-on occurring during the operation of the arrester).
The turn-on overvoltage ratio (K2):
K2=Vs6/Vs1=3.80/2.95=1.29. PA1 Kj=Tj6/Tj1=1.51. PA1 K2*=32 Vs6/Vs1=3.70/3.20=1.16. PA1 Kj*=Tj6/Tj1=1.20. PA1 VP : protection level of the arrester, PA1 Kt : test safety factor (1.15 in IEC standard), PA1 K1 : voltage distribution factor among series thyristors, and PA1 VDRM : a forward withstand voltage of the thyristor (a turn-on capability at this level is not guaranteed)
Junction temperature rise ratio (Kj):
In the case shown in FIG. 8 (in the case of turn-on without the arrester operation).
The turn-on overvoltage ratio (K2*):
Junction temperature rise ratio (Kj*):
Thus, K2&gt;K2* and Kj&gt;Kj*, and it is understood that the turn-on stress caused in the arrester operation is severer. Such severe turn-on stress may seem to be caused by the following reason. That is, the operation of the arrester equivalently provides a constant voltage source of VP connected between the terminals of the thyristor valve. If, in this state, the thyristor valve is turned on, rush current iL1, flowing from the power source side to the thyristor, is limited only by series-connected reactors (5-1, 5-2). In contrast, in the case under no arrester operation, rush current iL1 is limited by a series circuit of commutation reactance Lo and series-connected reactors (5-1, 5-2).
Therefore, according to a design for a thyristor valve having the conventional construction, the necessary number of series thyristors in the thyristor valve is determined as follows, with the aforementioned conditions (1) and (2) taken into consideration.
First, the number (N1) of series thyristors, necessary for satisfying condition (1), is determined as follows. EQU N1.gtoreq.VP.times.Kt.times.K1/VDRM (1)
where
The number (N2) of series thyristors necessary for satisfying condition (2) is determined as follows.
N2.gtoreq.VP.times.Kt.times.K1.times.K2/VDRM(ON) (2)
where VDRM(ON) (VDRM&gt;VDRM(ON)) is a maximum permissible thyristor voltage at which the thyristor can be turned on, and is a function of the rate of the rise of the current flowing into the thyristor (di/dt). FIG. 9 shows the relation between these VDRM(ON) and di/dt.
K2 is a turn-on overvoltage ratio.
The desired number of series-connected thyristors must be set to a larger one of N1 and N2.
From eq. (1) and (2), the following equation can be derived: EQU N2/N1=[VDRM/VDRM(ON)].times.K2 (3)
It is understood from eq.(3) that it is preferable to set VDRM/VDRM(ON) and K2 as close to 1.0 as possible from the viewpoint of the thyristor valve design, and it is ideal if N1=N2 is attained.
For this purpose, the series reactance is to be increased so that di/dt is reduced, and the thyristor is to be selected so as to suppress variation in the turn-on delay time. However, these manners increase the cost and there is a practical limitation.
For example, even when VDRM/VDRM(ON)=1.0, eq. (3) can be rewritten as N2=K2.multidot.N1.
Thus, the necessary number of series thyristors is set to be N2 so as to satisfy the condition (2). This means that the number of series-connected thyristors is equal to K2 times N1. According to the analysis described above, the series number must be set to a value equal to N1 times K2 (=1.29).
It is clearly understood from the aforementioned analysis that the number of series-connected thyristors cannot be determined at a low cost when the conventional thyristor valve is designed.